1. Field of the Invention
The present invention relates to a clinometric sensor, and more particularly relates to a clinometric sensor for finding an inclinational angle relative to the horizon. The present disclosure relates to subject matter contained in Japanese Patent Application No. 9-127137 (filed on May 16, 1997), which is expressly incorporated herein by reference in its entirety.
2. Description of the Related Art
Previously, as a clinometric sensor detecting an inclinational angle relative to the horizon, various types of sensors which output an electric signal corresponding to the position of a bubble in the liquid sealed in a closed container have been proposed. For example, in Japan Patent Laid-Open Publication No. Hei 7-146142, a clinometric sensor with a closed container assembled of a plurality of parts is proposed. The structure of this closed container is shown in FIG. 6.
In FIG. 6, a top board 60 has a disk shape wherein only a surface on one side thereof is shaped to be a spherical concave surface. On this spherical concave surface, upper electrodes 63, 63 with symmetrical shapes about the central axis of the top board 60 are formed. Furthermore, a bottom board 62 is made of a parallel flat glass shaped like a disc. On the whole area of the upper surface of the bottom board 62, a lower electrode 64 is formed. By affixing the top board 60 and the bottom board 62 to each end surface of a cylinder member 61, the closed container is completed. Then, in the closed container, the electrolytic solution L is injected and sealed, the remaining air forming a bubble B.
By using such a closed container, when voltage is applied between each of the upper electrodes 63, 63 and the lower electrode 64, the contact area of the electrolytic solution L to each of the upper electrodes 63, 63 is changed according to the position of the bubble B, so that the value of resistance between each of the upper electrodes 63, 63 and the lower electrode 64 may be changed. Consequently, the ratio of both currents flowing through the respective upper electrodes 63, 63 is changed. The ratio of currents is detected by a detection circuit (not shown in the figure) as a signal showing the inclinational angle relative to the horizon of the closed container.
The closed container thus arranged must be attached to the object of detection under the condition that the attachment accuracy in the detection direction (in which the inclinational angle is to be detected) is precise so that the inclination of the object of detection to which the closed container is attached may accurately correspond to the detection result. In other words, the relative attachment angle of the closed container and the object of detection in the detection direction (in which the inclinational angle is to be detected) must be adjusted so that the ratio of currents may be the value showing the horizontal state when any of the surfaces of the object of detection trends in the horizontal direction (or, when any axis of the object of detection trends in the vertical direction). On the other hand, the attachment accuracy in a direction rectangular to the detection direction (that is, the relative attachment angle of the closed container and the object of detection) does not directly affect the corresponding relation of the inclination of the object of detection and the detection result.
However, in the previous closed container with the above mentioned arrangement, the under surface of the top board 60 is formed as a spherical surface having a constant curvature in all directions and through the whole area. Therefore, when the closed container is inclined in the direction perpendicular to the detection direction, even if the inclination of the closed container in the detection direction is within the range in which the angle can be detected, there is a case where the bubble B may contact the wall surface of the cylinder member 61. Furthermore, if the angle of inclination of the closed container in the direction perpendicular to the detection direction exceeds the maximum angle capable of being detected by the clinometric sensor, the bubble B contacts the inner wall surface of the cylinder member 61 at all times. Since the tension for the bubble B to contact with the inner surface of the cylinder member 61 is larger than the buoyancy of the bubble B, the movement of the bubble B becomes unsmooth in case the bubble B has contacted the inner wall surface of the cylinder member 61. Therefore, if the bubble B has contacted the inner wall surface of the cylinder member 61, the angle of inclination of the object of detection cannot accurately be detected. In order to prevent such a problem, another clinometric sensor must be attached also in the direction perpendicular to the detection direction, with an accuracy approximately equal to that in the detection direction.
Furthermore, because of the above mentioned reasons, the closed container with the above mentioned arrangement can detect the inclinational angle of object of detection only in the range where the bubble B moves while contacting only the spherical concave surface of the top board 60. Accordingly, in the known closed container, in order to increase the range of the inclinational angle where the bubble B can properly move, a sufficiently large amount of electrolytic solution compared with the volume of the bubble B is injected and sealed in the closed container so as to make the diameter of the bubble B sufficiently smaller than the diameter of the spherical concave surface. Accordingly, even an insufficient change of the volume of the electrolytic solution L largely affects the volume of the bubble B. Therefore, depending on the temperature under the operating conditions of the clinometric sensor, there may be a case where the bubble B is expanded or shrunk to change the diameter thereof according to the change of the volume of the electrolytic solution L. In such a case, the proportion of the inclinational angle of the closed container to the contact ratio of the electrolytic solution L and each of the upper electrodes 63, 63 will fluctuate. Furthermore, if the volume of the bubble B becomes too small, the buoyancy of the bubble B becomes lower than the tension of the bubble B to contact with the spherical concave surface, so that the bubble B may not smoothly move according to the inclination of the closed container. Because of such a reason, when using the known closed container, there has been a case where the inclinational angle of the object of detection cannot accurately be measured.